Besides cellulose fiber, clay is the most widely used material in the paper industry. Filler clays are used in the wet end to decrease fiber costs. In the size press, coating clays are used to deliver specific properties: brightness, smoothness, and gloss. The main difference between filler and coating clays is particle size: in order to achieve smaller particle size for coatings, clays are delaminated to micron-scale particles. Because of this chemical alteration, coating clays are generally more expensive than filler clays.
Clays used for coating paper can be characterized by the percentage of particles by weight that are within a certain size range, for example, within 2 microns equivalent spherical diameter. More expensive materials have a higher percentage of these smaller particles. It has been recognized in the art that smaller particles, by virtue of their greater breadth, thinness and surface area, can provide a number of advantageous features. The coating is smoother with the smaller particles. Printability is increased because the larger surface area permits increased ink uptake. Brightness is enhanced because the multiplicity of fresh, unstained surfaces produces high reflectivity.
Crude clay deposits, such as Georgia kaolins, contain micrometer-size particles, ranging in size from about 0.1 microns to 15 microns. Particles at the smaller end of this range tend to be plate-like in shape, with diameters about 6 to 10 times thickness. Larger particles comprise stacks of particles that adhere in a face-to-face manner, like a stack of coins. Larger, granular clay particles can be broken down into smaller, delaminated particles by splitting the layered stacks into thin, platey units. Clay is a naturally occurring mineral in the phyllosilicate category, consisting of aluminum silicate as a principal component, along with various other metals such as calcium, potassium and magnesium, with varying levels of water content. Clays are formed from alternating sheets of tetrahedral SiO4 and octahedral AlO6, with the two sheets forming a layer. If a layer includes only one silica sheet and one alumina sheet, it is termed a 1:1 clay. Kaolin is an example of such a clay. These layers are tightly attached through hydrogen bonding. If, on the other hand, a layer is made of three sheets, as a silica-aluminum-silica sandwich, the clay is termed a 2:1 clay. Layers are attached to each other by van der Waals forces. There is a gap of about 1 nm between the layers, called the “gallery,” where various cations such as sodium, magnesium, calcium and lithium may reside. The smectite family of clays (montmorrilonite, hectorite, saponite) are 2:1 clays.
While delaminated clays offer advantages versus granular clays, especially for surface applications, both must be introduced into the papermaking process by using high loading volumes and high pressure calendaring. High clay loading leads to decreased mechanical properties such as Scott bond, Mullen, tensile strength and stiffness, as the clay mass interferes with the cellulose bonding in the paper product. High pressure calendaring is an energy-intensive process. There remains a need in the art to provide a clay additive product that offers the desirable features for finishing (smoothness, good ink uptake, brightness, etc.) at low additive levels with no inhibition of mechanical properties. It is further desirable that such an additive be deliverable in the size press as well as in a calendaring process. The size press application of clays leads to improved gloss, barrier properties and surface strength when the clay additive is used with a suitable binder. A smaller particle size, with increased surface area, can contribute to these advantageous properties.
Techniques to decrease particle size by breaking down the larger clay particles can be called delamination, while exfoliation processes break the clay particles into individual layers or clay platelets. Clays of interest for papermaking are typically classified either as kaolinite clays (1:1 clays, Kaolin as an example) and smectite clays (2:1 clays, Montmorillonites like Bentonite as an example). Traditionally, Kaolin clay is used as a filler in the wet-end to reduce cost of paper. Bentonite can be used as a microparticle retention system to enable high retention of fines, fibers and other additives in conjunction with a retention polymer.
Traditionally, the 1:1 clays, with their minimal interlayer distance, have been delaminated using mechanical means, such as ball mill grinding and extreme agitation. Following delamination, a polymeric dispersant must be used for the clay particles, which makes the retention of these particles difficult in the wet end. Moreover, the resulting particles are still not broken down to single-layer size: they are not fully exfoliated. The layers in the 2:1 clays are more readily separated, so are more amenable to exfoliation. For example, the intercalated cations allow for swelling within the gallery between the layers, so that the layers can be separated apart. Chemical or mechanical mechanisms can be used to exfoliate clays, with varying degrees of success.
Delaminated clays or exfoliated clays are highly desirable for this application owing to their high specific area and high aspect ratio. To improve the properties of clay additives, a variety of technologies have been utilized to improve upon the delamination process by detaching individual clay platelets from each other, a process termed exfoliation. Exfoliation can take place by physical or chemical processes to disrupt the attachment of the platelets to each other.
Traditional methods of exfoliation have utilized polymeric intercalating agents such as polyvinyl alcohol with swollen clay particles and then subjecting them to ultrasonication or similar high energy agitation. There remains a need, however, for quickly and inexpensively exfoliating the 2:1 clays to form stable suspensions for use in papermaking. Moreover, in papermaking, traditionally the clays are added either in the wet end or in the size press. Montmorillonite clays, for example, are traditionally used in wet-end as retention aids in conjunction with other polymers. For these uses, any exfoliated clay is desirably free of polymeric additives that might affect its interaction with the retention aids used in the papermaking process.
In addition, there remains a need to delaminate or exfoliate the clay particles into individual layers to enable better organization of clay platelets on paper surface leading to better barrier properties due to the tortuosity induced by the layered particles. The exfoliated clays also may reduce the need for supercalendering needed to orient and delaminate the clays resulting in a less dense but glossy sheet. There is also a need to keep the exfoliated clays suspended in aqueous solutions without using high molecular weight polymeric dispersants.